1. Field of the Invention
The present invention relates to an igniting device for a diesel engine having a thermal insulation structure made of a thermally insulating material such as ceramic or the like.
2. Description of the Related Art
There have been developed in recent years thermally insulated engines including various parts exposed to combustion gases. Elements such as a wall surface of a cylinder head which defines a combustion chamber, a piston head, intake, exhaust valves and exhaust ports are made of a thermally insulating material such as ceramic for higher thermal efficiency. A typical ceramic material used in such a thermally insulated engine is silicon nitride or the like, and which can withstand a high temperature of over one thousand degrees Celsius. The thermally insulated engine can operate even when the temperature of the wall of the combustion chamber reaches about 800.degree. C.
It is important that fuel to be used in diesel engines be well ignitable, and the ignitability of diesel fuel is indicated by a cetane number. A cetane number is represented by a ratio by volume of more ignitable cetane (C.sub.16 H.sub.34) to less ignitable .alpha.-methylnaphthalene (C.sub.11 H.sub.10). In Japan, the cetane number of light oil is about 55, and light oil having a cetane number of about 55 is used as fuel for ordinary diesel engines.
In a thermally insulated engine, the temperature of the wall of a combustion chamber is high. Since intake air introduced into the combustion chamber takes the heat of the combustion chamber wall, the temperature of the intake air as it is compressed in the combustion chamber is increased. If fuel having a cetane number of about 55, used for normal diesel engines, is used in such a thermally insulated engine, the fuel may be self-ignited resulting in so-called diesel knocking.
FIG. 3 is a graph showing the relationship between the crank angle and the temperature of a cylinder wall while a diesel engine is in operation. The graph has a horizontal axis representing the crank angle and a vertical axis indicating the cylinder wall temperature.
A solid-line curve in FIG. 3 shows the relationship between the crank angle and the cylinder wall temperature when fuel having a low cetane number is used in an ordinary diesel engine. The fuel is not combusted well because the ignitability of the fuel is poor.
In a thermally insulated engine, as indicated by the dotted-line curve in FIG. 3, air introduced into the cylinder takes heat from the high-temperature wall of the combustion chamber, and hence the temperature of the air as it starts to be compressed is high, so that the temperature of the air at the end of the compression stroke (0.degree. top dead center) is higher than that in the ordinary diesel engine. Consequently as seen in FIG. 3, the cylinder wall temperature of the thermally insulated engine is correspondingly lower at 0.degree. top dead center than the ordinary diesel engine. Therefore, the thermally insulated engine allows fuel, even if it is of a low cetane number, to be ignited.
When the thermally insulated engine is under a high load with the temperature of the combustion chamber wall reaching 600.degree. C., intake air is heated to such a temperature that fuel having a low cetane number can ignite. However, when the temperature of the combustion chamber wall is low at the time of starting the thermally insulated engine or operating the engine under a low load, the temperature of intake air cannot be increased to a point capable of igniting fuel. Thus, fuel of a low cetane number cannot be ignited.
In order to ignite fuel of a low cetane number under a low engine load, a glow plug used as a device for assisting in starting a diesel engine may be energized even in a low engine load condition for assisting in igniting the fuel. However, unless energization timing and an energizing current were controlled, supplied electric power would be wasted and the durability of the glow plug would be lowered resulting in early glow plug breakage.